As a security feature of information-processing equipments or the like, personal identification with the use of biometric information (biometrics) is known. In such personal identification, biometric information such as an image of the fingerprint or the like of a person to be identified is captured, and the characteristic information of the fingerprint is verified by using the captured image, thereby authenticating whether the person to be identified is truly him or her.
Biometric information is captured by a sensor such as a flat-type fingerprint sensor. Such a sensor is provided with, for example, a sensor plane (extracting plane) that is larger than a finger whose fingerprint is to be extracted, and biometric information may be captured by one or a few times of capturing processes.
However, as information-processing equipments having an identification function are downsized and compact information-processing equipments such as a mobile phone or a PDA (Personal Digital Assistant) are laden with such an identification function, it has become difficult to load such a large sensor as mentioned above. Hence, there are some techniques in which a sensor plane is made smaller than a finger and several partial biometric images successively extracted by that sensor plane are integrated to reconstruct a fingerprint image of the whole finger.
Such fingerprint sensors extract successive partial images by making finger move with reference to a sensor plane or by moving the sensor plane with reference to the finger, and such fingerprint sensors are referred to as, so called, sweep-type, slide-type, or swipe-type sensors.
As such a biometric information reading sensor, a technique in which a finger is slid with reference to a sensor and the image of a fingerprint is reconstructed from partial images of the captured biometric information (for example, see Patent Document 1), or the feature of providing an array of capacitive detecting elements for scanning fingerprint detection (see Patent Document 2) is known.
Regarding the process of combining partial images, a technique in which the amount of movement or the positions of movement of a finger are estimated from differences in the images obtained by sensors, thereby combining images to prevent a fingerprint image from expanding (see Patent Document 3), or a technique in which distortion is corrected according to the change in the moving speed of a finger (see Patent Document 4) is known.
Moreover, a technique is known in which a characteristic shape is extracted from partial images of a fingerprint, and partial images are combined according to the amount of movement of the images, which is detected from the consistency in the characteristic shape of temporally adjacent partial images (see Patent Document 5).
A technique is known in which an edge of the input fingerprint image is detected, and when the detected edge is largely different from specification values in which the values of edges are predetermined, a person who inputted the fingerprint image is notified of the elements to be corrected and is asked to input the fingerprint image again (see Patent Document 6).
Moreover, a technique is known in which the image quality of the obtained data of a fingerprint image is analyzed, and when the quality is poor, the reason why the quality is poor and the countermeasures are analyzed and the assessment results are announced visually or acoustically (see Patent Document 7).
Patent Document 1: Japanese Laid-open Patent Publication No. 10-091769
Patent Document 2: Japanese Laid-open Patent Publication No. 11-253428
Patent Document 3: Japanese Laid-open Patent Publication No. 2009-003564
Patent Document 4: Japanese Laid-open Patent Publication No. 2002-216116
Patent Document 5: Japanese Laid-open Patent Publication No. 2003-248820
Patent Document 6: Japanese Laid-open Patent Publication No. 2005-143890
Patent Document 7: Japanese Laid-open Patent Publication No. 2007-097842